1976 mercury 500 50hp thunderbolt classic boggs out

Johnoboat

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May 23, 2011
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I had my boat out on the lake and it fired up fine brought the idler down and she didn't stall. then when i go to accelerate I stall out. I found a sweet spot after many restarts where i could get it to run full open then shut it off and fish for awhile, then... same thing! It will start up fine, I get into gear and it wont stall if i don't gun it. But if i give it throttle it will stall. so i have to crawl home without getting the boat on plane, any thoughts?
 

hotrod53

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Oct 16, 2009
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

Sounds to me like carb issues, probably a piece of dirt in the high speed jet. Even if it's just one carb, the other cylinders would be trying to drag dead cylinders. If the motor can't get the fuel that it needs at high RPM, it will just bog. It sounds like your low speed jet will provide enough fuel to sustain a lower RPM, but you can't get flow thru the bigger jet to maintain a higher fuel demand.

It's all about fuel + air, as soon as you run out of one or the other, you get no more power. You're not going to run out of air, that's the carb butterfly opening, so you must be running out of fuel. I would start by pulling the carbs and giving them a good cleaning, or just rebuild them while they're off, kits are cheap. Your fuel pump could be the problem too, just rebuild them too while they're off. Remember, a carb kit does not include all of the parts that you need for the fuel pumps, they're two different kits and you will have parts left over.
 

weasel897

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Feb 7, 2012
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

Johno,

I'm having the same issue with my 78 Merc 500 50HP! I rebuilt the carbs and fuel pumps because it was running like crap. It ran fine right after doing all the work. Now, it's cutting out unless I baby the throttle. I saw on here where atwood bulbs were causing the engine to starve of fuel. I think I'll try to replace that and see where I get.

Weasel
 

Johnoboat

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May 23, 2011
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

That's the best explanation I've had yet!!! Ugh! Those carbs are hard to get at with my Chinese tools! Oh well... In I go!
 

hotrod53

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Oct 16, 2009
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

Do yourself a favor and remove the starter first, its almost impossible to do with it still on there and its only 4 bolts and a wire. Then remove the top carb first, try multiple box wrenches when trying to get at the nuts that are tight. I found that different wrenches can differ by a degree or two in their teeth making some wrenches better suited than others, even alternate them sometimes. Be careful with the carb sync linking the two carbs, its brittle. You can wrestle the choke linkage, if were ever going to break, I would have done it!

Johno, have you replaced your fuel line? The ethanol eats it right up and its cheap insurance.
 

michaud500

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Sep 3, 2011
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

I had the same issue! Not quit worked out all the kinks yet. I rebuilt the entire fuel system. I would go to WOT than back off and she would die and not start very well. Or when I floored it it would boog out ands die. So I adjusted the carbs. I backed the top carb out almost a full turn and she sprung into action. A LOT more power, I can floor it and she wont boog out. I still have a little bit of an issue though could be my ball pump. Seems to sucking for fuel but not enough there.

Hope that helps!
 

Johnoboat

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May 23, 2011
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Re: 1976 mercury 500 50hp thunderbolt classic boggs out

Is there a good starting position for high speed jets like a default setting? And what screw is the high speed jet?
 

hotrod53

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Oct 16, 2009
Messages
508
Re: 1976 mercury 500 50hp thunderbolt classic boggs out

Air screws are usually 1 1/4 to 1 1/2 turns out from the seat. The high speed jet usually sucks right out of the bowl of the carb, not a screw adjust. If you remove the carb and turn it upside down, you wil see the high speed jet screwed in from the bottom. While the carb is off, be sure even the tiny holes are clean, some are even hard to see when looking down the carb throat.
 
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